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Old clothes filter out cholera

13 January 2003

By Jenny Hogan

Using old saris to filter drinking water collected from rivers and ponds has halved the number of cholera cases in remote Bangladeshi villages.

“Many, many lives could be saved” by this cheap and simple way of reducing the cholera risk, says Rita Colwell, of the National Science Foundation (NSF) in Virginia, USA. Colwell led a three-year study, the results of which were published on Monday.

Any finely woven fabric would be an effective filter, so the technique could be applied around the world. Globally, cholera infection kills tens of thousands of people each year. It causes acute diarrhoea, leading to severe dehydration. The illness is endemic in impoverished regions of Asia, sub-Saharan Africa and South America.

In Bangladesh, people are forced to rely on cholera-contaminated surface water because many water wells dug in the 1970s and 1980s to provide clean water have since been found to contain high concentrations of arsenic.

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Optimum effect

The researchers found the optimum filtering effect when the sari fabric was folded so that the water passed through four layers of cloth. In laboratory tests, this removed more than 99 per cent of the cholera-causing bacteria, Vibrio cholerae. The bacteria themselves are tiny, but they cling to plankton that are too big to squeeze through the pores of the fabric.

The study showed that in practice this filtering reduces infections. The rate of new cholera cases in the 27 villages asked to filter water through saris was half that seen before the trial. And many of the cholera cases detected were traced to visits to communities not participating in the project.

Old clothes were more effective filters than new, because the fibres had slightly unravelled and this made the holes smaller. The cloth filters cut cholera cases as effectively as more expensive nylon filters designed to trap worm larvae, supplied by the World Health Organization.

Also encouraging, says Colwell, was the villagers’ response to the educational program. Once they were shown the bugs swimming around in the untreated water, they quickly realised the importance of filtering.

Precious fuel

However, Robert Quick of the US Centres for Disease Control and Protection told New Scientist&colon; “Our concern is that it won’t eliminate all contamination and people will still be at risk. We would recommend that people clean the water with chemicals, or boil it.”

He has worked with the World Health Organization to develop the “safe water system”, under trial in 15 countries. Filtering is only the first step of their system.

But in the regions of Bangladesh where the NSF has been working, disinfecting chemicals are hard to come by and there is so little wood that dried cow-dung is their only fuel, too precious to waste boiling water. Under such circumstances filtering water through old clothes “is, of course, better than nothing” says Quick.